Basic fuchsin-functionalized upconversion nanoparticles for the dual-modal detection of morpholine in fruit peels

Morpholine (MOR) is commonly incorporated into wax coatings applied to fruit surfaces to maintain freshness and extend shelf life. However, MOR can react with nitrite to form N-nitrosomorpholine, a potent carcinogen. Therefore, it is essential to monitor MOR residues in the peels. Herein, a dual-modal upconversion nanosensor based on the inner filter effect (IFE) was constructed for visual and quantitative detection of MOR in apple and citrus peels by combining upconversion nanoparticles (UCNPs) with basic fuchsin (BF). The UV-vis absorption band of BF overlaps with the emission spectrum of UCNPs, effectively quenching the upconversion luminescence (UCL) of UCNPs. Upon the addition of MOR, BF gradually faded, accompanied by the recovery of UCL from the UCNPs, leading to a dual-mode response to MOR based on changes in UCL intensity and the colorimetric signal. The detection limits (LODs) were evaluated to be 9.24 µM (S/N = 3) for the colorimetric mode and 0.95 µM (S/N = 3) for the UCL mode, respectively. Additionally, the constructed method exhibited good performances for the detection of MOR in actual fruit peel samples. The proposed nanosensor offers a new method to detect MOR in fruit peels, and possesses potential application in the field of food safety.